The present application relates to chemical separation methods and apparatuses.
In many fields, such as environmental monitoring and protection, airport security, food safety, and disease detection and diagnosis, it is often necessary to detect and identify the chemical compositions of an unknown sample. This task is often performed by first separating the different compounds in the sample, and then applying an identification technique to each isolated compound. One of the most popular methods for separating unknown compounds in a gas or liquid phase is called gas chromatography (GC), where the unknown sample is transformed into a carrier gas, if not already in the gaseous state, and the various compounds in the gas are separated due to their differing gaseous properties, such as polarity, affinity to the column and surrounding condition, etc. A standard method for isolating unknown compounds in a liquid phase is called liquid chromatography (LC).
Once the compounds are isolated, they may be identified. The simplest way to identify the compounds is by noting the retention time it takes for each compound to pass through the gas or liquid chromatograph, since different compounds correspond to different retention time to do so. But this method is limited to samples where much is known about the components and need pure standards to get their retention time at the same running conditions.
A more powerful method for identifying isolated compounds examines the intensity of different wavelengths of light emitted, transmitted, reflected, or scattered by the compound. This technique, called spectroscopy, works if each compound emits, transmits, reflects, or scatters light differently and if the spectroscopic instrument has sufficient spectral resolution to detect these differences. More specifically, different chemical compounds emit, transmit, reflect, or scatter different wavelengths of light with differing intensities. A graph or picture of such data is called the spectrum of that compound. Different types of spectroscopy reproduce the spectrum of a compound over different wavelengths and/or under different conditions. If the type of spectroscopy used provides a unique spectrum for each chemical compound, an unknown compound can be identified by producing its spectrum (for example, by illuminating the compound and measuring the light reflected, scattered, or emitted therefrom) and comparing its spectrum with the spectra of known compounds. As a result, gas or liquid chromatographs, which isolate compounds from a sample, are often used with spectrometers, which identify the compounds once they are isolated.
A challenge for gas or liquid chromatography is to provide a flexible and convenient device while still being able to perform the detection of the sample materials. Another challenge for gas or liquid chromatography is to have high sensitivity in the device such that a minute amount of the trace chemicals can be accurately detected.